The invention relates to bearing assemblies and, more specifically, to bearing assemblies having a lubricant disposed at the interface between the bearing insert and the bearing housing. The invention is also directed towards a lubricating grease that is particularly suitable for use in connection with such lubrication of bearing assemblies.
Various rolling element and sleeve bearings are known in the art. Rolling element bearings generally comprise a plurality of rolling elements, such as balls, disposed between inner and outer races of a bearing insert. In mounted rolling element bearings, the inner and outer races are disposed within a bearing housing, typically a pillow block housing. Sleeve bearings are similarly constructed, but in such bearings, the bearing insert comprises an inner sleeve and an outer sleeve that is disposed in a relatively rotating relationship with respect to the inner sleeve. Most mounted bearings in use today are of the self-centering variety, wherein the outer race or sleeve is not affixed to the housing, but rather is supported by the housing in a manner that permits limited pivotal rocking movement between the outer race or sleeve and the housing. Exemplary bearing assemblies are disclosed, for example, in U.S. Pat. Nos. 5,716,147 (Cook et al.), 5,863,137 (Johnson et al.), 5,704,719 (Cook et al.), and 5,927,864 (Feerick).
In such self-centering bearing assemblies, the housing and the bearing insert meet at an interface, which is typically defined by engaging curved profile portions of the insert and housing that allow relative pivotal rocking movement of the housing and bearing insert. One well known problem with such inserts is that the movement of the bearing insert with respect to the housing is difficult to initiate, but, once initiated, this movement proceeds undesirably rapidly, thus causing the overall movement of the bearing insert to be somewhat jerky. This problem, known in the art as the xe2x80x9cstick-slipxe2x80x9d problem, is of importance both in connection with the installation of the bearing onto a machine, and in connection with the calibration of the bearing assembly during manufacture. The stick-slip problem is of particular significance in connection with the calibration of bearing assemblies. If the housing fit about the insert is too loose, the bearing may experience excessive fretting wear during its service, especially when vibratory load is encountered. On the other hand, if the housing fit is too tight, large housing torques, or resistant moments resulting from the friction between the bearing insert and the housing, are required to effectuate self-alignment. Bearing assemblies are calibrated using a tool such as a torque wrench which measures the breaking housing torque, or torque required to initiate relative movement of the bearing insert and housing, and moving housing torque, or torque required to maintain relative movement of the insert and housing. The stick-slip problem can make these torque measurements difficult to obtain.
In overcoming these problems, there is now provided a bearing assembly that includes a lubricant disposed at the interface between the bearing insert and housing, which lubricant comprises a lubricating material that provides sufficient lubrication between the housing and insert such that the breaking housing torque is no greater than 130% of the moving housing torque, and preferably is less than the moving housing torque. It has been found that fatty acids are particularly suitable as lubricating materials to provide such relationship between the breaking and moving housing torques. In a highly preferred embodiment of the invention, the fatty acid is provided in the form of a grease which comprises a base lubricating oil and a fatty acid thickener that is substantially uniformly dispersed therein.
These and other features and advantages of the invention will be more readily apparent upon reading the follow description of preferred exemplified embodiments of the invention and upon reference to the accompanying drawings, wherein: